1. Field of the Invention
The present invention relates to dispersion strengthened aluminum-base alloys, and more particularly to a method for reducing the gas content of an extruded, forged or rolled aluminum powder metallurgy product.
2. Description of the Prior Art
In recent years the aerospace industry has searched for high temperature aluminum alloys to replace titanium and existing aluminum alloys in applications requiring operating temperatures approaching 350.degree. C. While high strength at ambient and elevated temperatures is a primary requirement, certain design applications mandate that candidate alloys also exhibit, in combination, ductility, toughness, fatigue and corrosion resistance, as well as lower density than the materials currently being used.
One of the major restrictions to the widespread utilization of high temperature aluminum alloys is their inability to be welded or brazed. The application of standard welding and brazing practices to these high performance aluminum alloys results in the formation of excessive porosity in the weld and heat affected zone of the joint due to the outgassing of the alloy during the joining cycle and the coalescence of the gases to form porosity. The excessive gas porosity, is caused in part by the presence of hydrogen, as hydroxide or water, in the base metal. Also the slow cooling of the welded area may favor the formation of coarse, brittle intermetallics which will severely reduce the joint strength and ductility when compared to the base metal. Finally, any treatment given to these alloys to improve their weldability must be cost effective.
The hydrogen content may be reduced by heat treatment of the high temperature aluminum alloy in vacuum at high temperature. However, the heat treatment is limited by the reduction of the base metal strength as the heat treating time and temperature increases. Previous disclosures have shown that the weld porosity in powder metallurgy aluminum alloys (Al-10Fe-5Ce) can be virtually eliminated by a combination of preweld vacuum heat treatment, i.e. 750.degree. F. for 24 hrs. in vacuum, and direct current electrode negative welding, with only a minor decrease in base metal tensile strength, the welds exhibit a brittle behavior due to brittle phases formed near the weld interface. These welds are restricted to non-structural applications. (Gas Tungsten Arc Welding of Al-10Fe-5Ce, Guinn Metzger, report No. AFWAL-TR-87-4037, AFWAL/MLLS, Wright-Patterson AFB, Ohio 45433, February 1987).
To date, the majority of aluminum base alloys being considered for elevated temperature applications are produced by rapid solidification. Such processes typically produce homogeneous materials, and permit control of chemical composition by providing for incorporation of strengthening dispersoids into the alloy at sizes and volume fractions unattainable by conventional ingot metallurgy. Processes for producing chemical compositions of aluminum base alloys for elevated temperature applications have been described in U.S. Pat. No. 2,963,780 to Lyle et al., U.S. Pat. No. 2,967,351 to Roberts et al., U.S. Pat. No. 3,462,248 to Roberts et al., U.S. Pat. No. 4,379,719 to Hildeman et al., U.S. Pat. No. 4,347,076 to Ray et al., U.S. Pat. No. 4,647,321 to Adam et al. and U.S. Pat. No. 4,729,790 to Skinner et al. The alloys taught by Lyle et al., Roberts et al. and Hildeman et al. were produced by atomizing liquid metals into finely divided droplets by high velocity gas streams. The droplets were cooled by convective cooling at a rate of approximately 10.sup.4 .degree. C./sec. Alternatively, the alloys taught by Adam et al., Ray et al. and Skinner et al. were produced by ejecting and solidifying a liquid metal stream onto a rapidly moving substrate. The produced ribbon is cooled by conductive cooling at rates in the range of 10.sup.5 to 10.sup.7 .degree. C./sec. In general, the cooling rates achievable by both atomization and melt spinning greatly reduce the size of intermetallic dispersoids formed during the solidification. Furthermore, engineering alloys containing substantially higher quantities of transition elements are able to be produced by rapid solidification with mechanical properties superior to those previously produced by conventional solidification processes.
The need remains in the art for a process for reducing the gas contents of rapidly solidified, dispersion strengthened aluminum base alloys while retaining useful mechanical properties after welding or brazing.